This invention relates to fossil fuel-fired steam generators and in particular to a steam flow path arrangement for covering the walls of the rear gas pass.
In a large fossil fuel-fired steam generator fuel is fired into a vertically extending furnace and steam is generated in the tubes lining the walls of the furnace. Gas leaving the top of the furnace passes horizontally to a rear gas pass and then passes downwardly through the gas pass as it travels toward the stack. Within the horizontally flowing portion and in the downwardly flowing rear pass, tubular heat exchange surface is supported. This heating surface is used to superheat the steam and to reheat the steam after an initial traverse through the portion of the turbine. Economizer heating surface is also normally suspended at the bottom of the vertical gas pass.
The gas flowing through this gas pass tends to be at a temperature level higher than that of the saturated steam, and it is conventional to cool the walls by lining them with tubes carrying saturated steam between the drum and the superheater surface. These tubes not only line the wall for cooling purposes but also act as support members for the walls themselves and other pressure and nonpressure parts.
Heat transfer to the walls is normally relatively low since there is little radiation at the low gas temperature levels and the tube-covered wall does not have high convection heat transfer characteristics. The tubes lining the front wall of the rear gas pass, however, pass upwardly through the gas inlet to a support elevation above the roof, and thereby are exposed to convection heat transfer where the gases flow across the tubes. It is important that these tubes be at relatively uniform temperatures since any particular hanger tube becoming hot will expand and tend to release the load carried by the particular tube so that other tubes are subject to increased loading. Furthermore, a hot tube which releases its load is subject to less tension and is more likely to vibrate because of the flue gases passing thereover. It is, accordingly, important that these tubes operate at the same temperature as one another.
Downflow through these tubes produces a tendency toward unstable flow since a tube which starts to become hotter has less dense steam therein. This results in increasingly less flow in the particular tube; and as a result, the steam and the tube become even hotter, with the flow tending to decrease even further. With an upflow situation any overheating of the tube has a tendency to increase the flow and is, therefore, self-stabilizing.
Inlet headers for superheaters or reheaters must be supported within the gas pass; and where the superheater inlet header cannot be supported in the steam-cooled wall itself, complex support arrangements are required.
Various tubing arrangements are also subject to erosion because of the fly ash contained within the gases. This is particularly difficult immediately adjacent the walls where gas at high velocity tends to bypass the heating surface. Where a header located in the wall is required to receive tubes from above and to also supply tubes from above, the large number of openings in the header requires that some of these tubes be bent into the gas pass, thereby aggravating the erosion problem.
Futhermore where tubes must pass down the wall for cooling the walls and then must flow from a lower header to superheater surface, double wall tubing is required which increases the cost of tubing and also requires additional support arrangements to hold the tubing back to the wall.